Add deselecting-item callback to StandardListView

[Enyium]

With StandardListView's current-item-changed callback, you can currently only be informed of the new selected item after the fact.

As described in #6369, I'm saving widgets' content in my app after user actions subsided for a short duration (using a Timer). When the list item changes, I have to save immediately, for which I would call the one-shot Timer's prepone() function as described in the linked issue.

For this all to integrate as best as possible, StandardListView would need to get an additional callback deselecting-item that would be called before current-item changes, and only if the item is guaranteed to be deselected. This way, I'd call Timer's prepone() in deselecting-item, which would call Timer's triggered (if the Timer is running), which would read StandardListView's current-item to save the widgets' content for this list index.

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Perhaps this should also be implemented for StandardTableView.